Roadways throughout the United States often must be constructed through mountainous or hilly terrain. Typical construction methods for these types of roadways involves blasting rock away from the uphill side of the mountain and using this blasted rock and soil in the construction of the road. The blasted rock fill is often placed on top of steep terrain and used to form the roadway. When the fill becomes unstable and slides away from the roadway, it forms a landslide. Landslides are dangerous and problematic in that they can disrupt the normal flow of traffic along these roadways, which are often major thoroughfares.
When such landslides occur, they must be stabilized in order to return the roadway to normal, usable conditions. In order to stabilize the landslide, a tiedback retaining wall is often constructed. Permanent soldier beam and tieback retaining walls are known in the art and have been constructed to stabilize landslides or for other earth retaining purposes.
Conventional permanent earth retaining walls for landslide stabilization use large-diameter drilled shafts for soldier beams. The soldier beams are formed in an open drilled shaft or they are formed in a cased drilled shaft. These conventional soldier beams either consist of a steel beam placed in the drilled shaft and surrounded by a cementaceous material or they consist of a reinforcing bar cage placed in the drilled shaft and filled with structural concrete. If casing is used, it is removed after the cementaceous material of concrete is placed. The direct interaction between the cementaceous material or structural concrete and the ground enhances the soldier beam's bond to the ground.
When these conventional soldier beams are installed in caving ground with hard boulders, such as is found in landslides in mountainous terrain, special drilling methods must be used to install the drilled shaft. These special drilling methods are commonly known as overburden, percussion drilling methods, which permit a casing to be advanced as the shaft is drilled. They do so by drilling a hole having a larger diameter than the casing. A structural steel soldier beam is set inside the casing and the hole is backfilled with cementaceous material or structural concrete. Then the casing is removed leaving a steel soldier beam in a cementaceous material filled hole in direct contact with the ground. Another method of creating the soldier beam is to insert a reinforcing bar cage into the cased hole and filling the casing with structural concrete. Then the casing is removed leaving a structural concrete soldier beam in direct contact with the ground. After the casing is removed, the large drilling equipment is moved to another soldier beam location.
Installing large diameter soldier beams requires overburden percussion drilling equipment that is large enough to advance a cased drill shaft 24 to 36 inches in diameter or larger. The difficult ground conditions and large equipment required to support such drilling make constructing these types of retaining walls expensive and time consuming. In addition, such large size drilling equipment requires a considerable amount of space in which to set the equipment while constructing the wall. Often, landslides requiring retaining walls occur on major roadways that have limited space in which to work. The issue of limited space is compounded when the roadway must remain open to traffic, even if only to limited traffic flow. Often, using such large diameter equipment necessitates complete or partial road closure, which causes severe traffic flow disruptions.
Tiebacks have been used with conventional soldier beam retaining walls to provide lateral support to the wall. The tiebacks of conventional permanent retaining walls with concrete faces are directly attached to the vertical soldier beams. In some applications, the drilled shafts are installed from an elevation near the bottom of the wall and the permanent tiedback earth retaining wall is constructed via bottom-up methods. Bottom-up methods require installation of the facing and backfill behind the wall from the bottom of the wall toward the top of the wall.
Piles having a relatively small diameter, known as micropiles, are known to be used for the stabilization of landslides in non-retaining wall circumstances. In one conventional configuration, they generally stabilize a slide without forming a retaining wall. In another configuration, tiebacks, installed in a common cap beam with the micropiles, are used to provide additional lateral support for the micropiles.
In another conventional configuration, a large number of individual micropiles are applied to unstable ground so that it functions as a united mass. In this configuration the micropiles are small diameter, low structural capacity micropiles formed of a grouted bar rather than a steel casing. The mass of the reinforced ground with the large number of micropiles can be large enough to function as a gravity earth retaining structure, without an exposed structural wall face.